This application is the national phase under 35 U.S.C. xc2xa7371 of PCT International Application No. PCT/DE99/00614 which has an International filing date of Mar. 8, 1999, which designated the United States of America.
Gas turbine burner and method of reducing combustion oscillations during operation of such a burner.
The invention relates to a gas turbine burner. It also relates to a method for reducing combustion oscillations during the operation of such a burner.
The formation of combustion oscillations can occur during an operation of a gas turbine burner. This is also known under the concepts of xe2x80x9ccombustion chamber hummingxe2x80x9d, xe2x80x9ccombustion chamber oscillationsxe2x80x9d, xe2x80x9ccombustion-induced pressure pulsationsxe2x80x9d, xe2x80x9coscillating combustion processesxe2x80x9d. The combustion oscillations are due to an interaction between the quantity supplied per unit time of combustion air/fuel mixture flowing in the flow duct of the burner. The mixture is ignited after entry into a combustion chamber and burns in a flame, with the momentary combustion conversion in the flame. Combustion conversion designates the quantity of combustion air/fuel mixture converted per unit time during a combustion process in a flame. Pressure fluctuations in the combustion chamber, which can lead to the formation of a stable pressure oscillation, can occur due to a change in the combustion conversion. In addition to an increased production of noise, the combustion oscillations cause an increased mechanical and thermal load on walls associated with the combustion chamber and on other parts belonging to the combustion system.
A hybrid burner for a gas turbine is known from EP-0 580 683 B1. A hybrid burner has both a diffusion burner and a premixing burner. It can therefore be operated both in diffusion operation and in premixing operation. The premixing burner of the hybrid burner has an annular duct for the supply of combustion air or a combustion air/fuel mixture. Arranged in the annular duct, there is a swirl device for imposing a swirl on a flow, which is formed by the combustion air/fuel mixture supply or the combustion air in the annular duct. This swirl device is also designated as a swirl cascade. The diffusing burner of the hybrid burner is arranged coaxially within the annular duct of the premixing burner. The diffusion burner has a combustion air supply duct, which is designed as an annular duct and in which a fuel supply duct is coaxially arranged. The ducts of the diffusion burner open into a nozzle. In addition, the diffusion burner has a pilot burner, which is only necessary for the operation of the premixing burner, in its combustion air supply duct.
During a premixing operation of the hybrid burner, a combustion air/fuel mixture is supplied via the annular duct of the premixing burner and this mixture forms a flow, on which a swirl is imposed by means of the swirl cascade, in the annular duct. The swirling flow emerges from the premixing burner into a combustion process. The combustion process is stabilized by means of the pilot burner flame. During a diffusion operation of the hybrid burner, combustion air and fuel are respectively supplied via the combustion air supply duct and the fuel supply duct to a mixing process in the region of the nozzles of the diffusion burner. The combustion air/fuel mixture formed during the mixing emerges from the diffusion burner into a combustion process.
The object of the invention is to provide a gas turbine burner with a smaller tendency toward the formation of combustion oscillations. In addition, a method of reducing combustion oscillations in such a burner is to be provided.
In accordance with the invention, the object directed towards the gas turbine burner is achieved by the features of claim 1. Advantageous embodiments are the subject matter of subclaims which refer back to claim 1.
The invention is based on the knowledge that during an operation of such a burner in the flow duct, a flow of combustion air is formed to which fuel is admixed via fuel inlets for the formation of a combustion air/fuel mixture. The supply of fuel preferably takes place into a flow of combustion air which is uniform over the cross section of the flow duct. This has the advantage that the local mixture ratio of combustion air to fuel is essentially uniform over the cross section. The NOx content of the exhaust gas, which occurs due to the combustion of the combustion air/fuel mixture formed during the mixing process, can be influenced by a uniform mixture. Before the combustion air/fuel mixture is supplied to the combustion process, a swirl is preferably imposed on the flow in the flow duct by means of a swirl device in order to stabilize the combustion process. This swirl is usually uniform over the periphery of the flow duct in order not to impair the uniformity of the flow. Investigations have shown that as the flow of the combustion air/fuel mixture supplied to the combustion process by means of a burner becomes more uniform, the probability of the formation of combustion oscillations occurring during an operation of the burner in a combustion system increases.
The invention is therefore based on the idea of designing a gas turbine burner in such a way that the flow of a combustion air/fuel mixture which has been formed is made sufficiently non-uniform, by imposing a non-uniform swirl before the supply to a combustion system, for the excitation of combustion oscillations during an operation of the gas turbine burner in a combustion system to be at least substantially reduced. For this purpose, a swirl which is non-uniform in the peripheral direction is imposed on the flow of the combustion air/fuel mixture by means of a swirl device arranged in the flow duct.
Although it is known, from U.S. Pat. Nos. 5,388,536 and 5,415,114, to arrange vane-type deflection elements in the flow duct of a burner with different setting angles or at different spacings from one another, the intention of this was to create fuel-weak and fuel-rich flow duct regions in burners for steam generators in order to reduce the NOx content in the combustion gas.
The swirl device preferably has a plurality of swirl elements, each swirl element having a deflection surface. A main flow direction of the flow of the combustion air/fuel mixture, which can change as a function of the burner geometry, is defined on the basis of the geometry of the flow duct, which is configured as an annular duct. The deflection surfaces of the swirl elements each have an outlet angle, relative to the main flow direction, at the outlet end which is preferably different in the case of at least two directly adjacent swirl elements. This achieves the effect that after flowing through the swirl device, the flow emerges at two different angles, at least, in the peripheral direction, so that the swirl imposed is non-uniform in the peripheral direction. Also preferred is the configuration of each swirl element as a swirl vane. The burner can, for example, be configured as a hybrid burner of the gas turbine.
A plurality of swirl elements with the same outlet angles preferably form a swirl group and the swirl device has at least one such swirl group. The formation of the swirl device from a plurality of swirl groups is also preferred, with adjacent swirl groups having different outlet angles. The swirl device can then be formed from six swirl groups, for example, it being possible for each swirl group to have four swirl elements.
In order to reduce the quantity of the NOx compounds formed during a combustion process, it is desirable to supply fuel into a uniform flow of combustion air for the formation of a combustion air/fuel mixture intended for combustion. For this purpose, the burner preferably has a fuel inlet through which fuel can be supplied into the flow duct upstream of the outlet end. In this way, the fuel is supplied, before the swirl device, into a uniform flow of combustion air flowing in the supply duct.
The object directed toward the method is achieved, in accordance with the invention, by the features of claim 6. In this, a flow of a combustion air/fuel mixture or of combustion air has a swirl, which is non-uniform in the peripheral direction of the annular duct, imposed on it in a flow duct, which is configured as an annular duct, of the gas turbine burner.
The swirl device for imposing the swirl and the method for reducing combustion oscillations are explained in more detail by means of the embodiment example represented in the drawing.